1. Field of the Invention
This invention relates to a telephone system and, in particular, to a private branch exchange (PBX) system having equipment for automatically providing return answer supervision on tie trunk calls between two PBXs.
2. Description of the Prior Art
It is common practice to provide supervisory circuits of various types and configurations in PBX systems in order to implement useful and valuable customer services and features. One such particular feature is desired when a call to a local PBX requests a station located in a distant PBX. An attendant located at the local PBX receives the call on an incoming central office trunk and then transfers the call over a tie trunk to the distant PBX.
Prior art arrangements provide a return supervisory indication to the attendant when the common control of the local PBX establishes a network connection between the calling central office trunk and an idle tie trunk. This return supervisory indication activates a previously unlit lamp on the attendant's console to indicate that the network connection is made and that the calling line is transferred to an idle tie trunk. If the lamp remained unlit, the attendant could further service the calling party in the event, for example, the tie trunks are all busy.
Although prior art arrangements operate satisfactorily to provide supervision to the point of the network connection, the call is denied attendant supervision on tie trunk calls after the tie trunk is seized. For example, when the called party does not answer, in prior art arrangements, the calling party must place a second call to the local PBX attendant if the calling party desires further service.